This structure is able to be used mainly in telemetric and optical telecommunications applications.
Integration eliminates the defects of hybrid circuits; for example, in an optical fiber hybrid circuit, it is difficult to adjust optical access paths with a pair of equilibrated photodiodes of more than 1 mm for flowrates exceeding 600 mega binary elements per second, these paths being required for coherent heterodyne or homodyne reception.
The first integrated monolithic guide photodetector structures were made of silicon. In this technique, the light guides are made of a dielectric material, generally deposited on SiO2 and detection is ensured by a diode embodied in the substrate. Such a structure is described in the document by D. B. Ostrowsky and al and entitled "Integrated optical photodetector", Appl. Phys. Lett., vol. 22, No 9, May 1973, p. 463.
This solution is unable to be transposed to semiconductive structures made of III-V or II-VI materials since the semiconductive material guides have the same ranges of absorption, and thus transparence, as the substrate.
In this technology, a mixed guide photoconductor structure containing dielectric films and III-V material is described in the document by J. C. Gammel and J. M. Ballantyne and entitled "An integrated photoconductive detector and waveguide structure", Appl. Phys. Lett. 36 (2), January 1980, p. 149. In this structure, the optical guides are still made of a dielectric material and not of a semiconductive material.
The article by R. Trommer entitled "Monolithic InGaAs photodiode array illuminated through an integrated waveguide", Electronic Letters, vol. 21, No 9, April 1985, p.82, refers to the first photodetector integrated with a III-V material-based optical guide. This integrated structure has a certain number of drawbacks.
In particular, the embodiment of this structure requires two epitaxial growths, one on the front face and one of the rear face, this embodiment thus proving to be a complex one. The luminous beam to be detected traverses a thick film of InP n+ (about 200 micrometers) which attenuates one portion of the beam due to the absorption of the latter by the free carriers. Finally, a rear V-shaped section needs to be made to coincide with the front photodetector opposite, which presents significant embodiment difficulties.
A certain number of integrated guide photodetector structures made of III-V material via the coupling by dying out waves has been envisaged (see, for example, the article by C. Bornholdt and al and entitled "Waveguide-integrated p-i-n photodiode on InP", Electronic Letters, vol. 23, No 1, January 1987, p.2 or the article by S. Chandrasekhar and al and entitled "Monolithic integrated waveguide photodetector", Electronic Letters, vol. 39, No 10, May 1987, p. 501).
In the current state of embodiments, the coupling lengths need to be considerable, that is several hundreds of micrometers, so as to have the photodetector zone carry out absorbtions exceeding 90%. It thus follows that the dimensions of the photodiodes be large, hence resulting in large impedances and reduced response speeds, as well as higher dark currents.
Various integrated guide photodetector structures made of III-V material with butt coupling have been envisaged (see the document by S. Chandrasekhar and al and entitled "Integrated waveguide p-i-n photodetector by MOVPE regrowth", IEEE Electron device letters, vol. EDL-8, No 11, November 1987, p. 512). In this type of structure, the diffusion area p of the diode p-i-n needs to be localized with a large amount of precision with respect to the extremity of the optical guide (precision less than 1 micrometer). This represents the major problem posed by butt coupling.
In addition, one portion of the light to be detected may pass outside the absorbant film and thus reduce detection efficiency. In addition, the structure is not planar; as a result, diffusion p on the mesa results in a squeezing of the intrinsic zone at the level of the guide and thus a risk of the diode breaking down.
In the article by W. Doldissen and al ("Butt coupled photodiodes integrated with Y-branched optical waveguides on InP", Electronic Letters vol.25, No 1, Jan. 1989, p.35), the butt-detection of light is ensured by a diode obtained by a diffusion p+ above the guide film, the surface of the diode then being delimited via chemical attack in mesa form. The exposed junction is then rendered passive by a suitable nonconductor. This technique has never been the best technology for the time-behaviour of the diffused diode. According to the inventors, a planar diode obtained by the localized diffusion of the doper p in a film n does not improve the situation.